1. Field of the Invention
Embodiments of the invention described herein pertain to the field of submersible pumps. More particularly, but not by way of limitation, one or more embodiments of the invention enable an apparatus, system and method for sealing submersible pump assemblies.
2. Description of the Related Art
Electric submersible pump (ESP) assemblies are used to artificially lift fluid to the surface in deep underground wells such as oil, water or gas wells. Exemplary downhole oil well fluid, for example, may include a mixture of oil, water and natural gas. A typical ESP assembly is shown in FIG. 1, consisting of electric motor 100, conventional seal section 110, pump intake 120 and centrifugal pump 130, which are all connected together with rotatable shafts. Electric motor 100 supplies torque to the shafts, which provides power to pump 130.
Submersible pumps operate while submerged in the fluid to be pumped. The fluid enters the assembly at pump intake 120 and is lifted to the surface through production tubing 140. In order to function properly, electric motor 100 must be protected from well fluid ingress, and conventional seal section 110 provides a barrier to keep the well fluid from the motor and its motor oil. In addition, conventional seal section 110 supplies oil to the motor, provides pressure equalization to allow for expansion of motor oil in the well bore, and carries the thrust of pump 130 through the use of thrust bearings. A conventional multi-chamber seal section is further illustrated in FIG. 11. Conventional seal section 110 of FIG. 11 includes conventional head 1125, three conventional seal chambers 1130, a conventional thrust chamber 1120 and conventional base 1135. Conventional seal chambers 1130 are attached to one another and conventional thrust chamber 1120 by barstock guides 1155. As illustrated in FIG. 11, conventional thrust chamber 1120 is located at the bottom-most section of conventional seal chamber 110 and connected to motor 100 by conventional base 1135.
In many instances, naturally occurring sand is pulled into the pump assembly along with the well fluid and can accumulate in production tubing 140. When the pump is shut down, the sand may fall back down through the pump assembly and accumulate in conventional head 1125, at the top of seal section 110, which is traditionally open to the accumulation of debris, and includes conventional mechanical seal 1110 and conventional vent port 1105. As shown in FIG. 11, sand can accumulate at the top of conventional mechanical seal 1110 due to conventional seal section 110's open design, destroying mechanical seal 1110.
This accumulation of sand may also plug the conventional vent port 1105, which vents to conventional mechanical seal 1110. Vent ports function to provide an outlet for expanding motor oil into the well bore, in order to maintain equalized pressure. Pressure equalization may be accomplished by utilizing a u-tube or elastomeric bag design. In either case, the expanding oil is released through an internal check valve located inside conventional vent port 1105. If the vent port is blocked off by sand, conventional seal section 110 cannot equalize pressure, causing a pressure build up inside conventional seal section 110, such that the mechanical seal 1110 faces may eventually separate. If this occurs, well fluid and sand will enter the clean oil section of conventional seal section 110 (upstream of conventional mechanical seal 1110), impeding the seal's proper function which may lead to failure of the pump.
Accumulation of sand may also prevent well fluid from making contact with the faces of mechanical seal 1110 of conventional seal section 110. Mechanical seal 1110 faces must be in contact with well fluid to remain cool during operation. In the instance that sand compacts around the mechanical seal and prevents heat transfer with the well fluid, the sealing faces will overheat and cause failure of the seal whether or not the vent port is plugged. In addition, conventionally a bronze bushing (not shown) is located in conventional head 1125, just below the mechanical seal, to provide radial support. Well fluid contamination and sand will rapidly destroy the bushing, causing a catastrophic failure due to loss of radial shaft support.
As is apparent from the drawbacks of conventional designs, seal sections of submersible pump assemblies are unduly susceptible to damage and contamination by sand and well fluid. One conventional approach to address this drawback has been to add a plate over the top of conventional head 1125. Such plates capture a portion of sand that would otherwise fall into the seal section, but they also prevent cooling well fluid from exchanging heat with the mechanical seal. In addition, plates over the seal section do not adequately prevent sand from entering, as they are prone to leaks.
Another approach to address this drawback has been to include multiple seal chambers in order to provide redundancy. As shown in FIG. 11, three conventional seal chambers 1130 are included in conventional seal section 110. In multiple chamber designs, thrust bearings are conventionally located at the bottom most section of the seal assembly, close to the motor in conventional thrust chamber 1120. In FIG. 11, a conventional upthrust bearing 1150, conventional thrust runner 1145 and conventional downthrust bearing 1140 are included in conventional thrust chamber 1120. As shown in FIG. 11, conventional thrust chamber 1120 is in close proximity to motor 100. With the multi-chamber approach, if one chamber should fail and allow well fluid to enter that chamber, the succeeding chamber will still isolate well fluid and the conventional bearings 1140, 1150 remain protected from contamination until the last chamber is breached. However, the result of the multi-chamber designs is that the shaft is very long and slender, which may cause incipient buckling. If this occurs, the side load capacity of the bronze bushings may be overcome as the shaft tries to buckle, causing pump failure.
Additionally, the location of conventional downthrust thrust bearing 1140, conventional thrust runner 1145 and conventional upthrust bearing 1150 in close proximity to the motor exposes the bearings to excessive amounts of heat. The conventional thrust bearings 1140, 1150, traditionally located at the bottom-most section of the seal assembly, sit immersed in clean motor oil to handle the thrust of the pump. Thrust bearings in the seal section carry the axial thrust and maintain shaft alignment. Hydrodynamic bearings are the most commonly implemented thrust bearings in submersible pump applications.
A conventional hydrodynamic bearing includes two round disks, which are usually submerged in a cavity of clean motor oil. One disk is fixed, while the other is turned by the shaft in rotation about the central axis of the fixed disk. An exemplary conventional thrust bearing of the prior art is illustrated in FIGS. 12A and 12B. Conventional downthrust bearing 1140 is illustrated in FIGS. 12A and 12B, but traditionally, conventional upthrust bearing 1150 would be identical except installed in conventional seal section 110 facing in the opposite direction of conventional downthrust bearing 1140. In some approaches, the fixed disk (conventional downthrust and upthrust bearings 1140, 1150) is designed with bronze pads. The rotating disk pulls motor oil between the pads and the stationary disk. As long as there is motor oil between the surfaces, the thin film of fluid creates separation between the disks with hydrodynamic lift. To function properly, the surfaces of hydrodynamic bearings must be flat and smooth. A typical hydrodynamic thrust bearing is usually designed to operate with a fluid thickness of between about 0.001 and 0.0004 inches. Any impurities that are thicker than the oil film between the disks, such as sand in the motor oil, can cause surface damage to the bearings. Resulting friction between the disks reduces or eliminates their hydrodynamic properties. Contamination of the motor oil between the disks, for example with sand, is common due to typical oil field conditions and oil or water pump requirements. Placing the disks in a protected cavity usually means locating the disks closer to the motor, exposing the disks to increased heat.
The rotating disk of a hydrodynamic thrust bearing is typically a hard material such as tungsten carbide. The stationary disk, conventional downthrust bearing 1140 and conventional upthrust bearing 1150, typically include softer metal pads made of bronze. However, bronze is only capable of carrying a load of about 500 pounds per square inch. There is often insufficient space to include large enough copper pads on the stationary disk to carry the required loads.
Conventional thrust bearings are not well suited for submersible pump applications since they must be operated in a cavity of clean motor oil uncontaminated by sand, dirt or water. In submersible pump applications where solid laden fluid is pumped, this means placing the thrust bearings close to the motor in a cavity of clean motor oil, which is not an ideal location for carrying thrust and maintaining shaft alignment.
Thus, it is apparent that conventional sealing techniques do not satisfactorily provide protection from sand contamination in submersible pump assemblies. Therefore, there is a need for an additional apparatus, system and method for sealing submersible pump assemblies.